1. Field of the Invention
The present invention relates to a slit switching mechanism for spectrometers.
2. Description of the Prior Art
An example of a prior art spectrometer slit switching mechanism for switching entrance and exit slits is described in Japanese Laid-Open Patent Publication No. Sho-57-67826. As shown in FIG. 9 of the present application, this mechanism is comprised of a circular rotation plate 3 connected to an output shaft 2 of a stepping motor 1. In this structure, the circular rotation plate 3 has pairs of equally spaced entrance slits 4 and exit slits 5 formed at a prescribed radial distance from the center of the plate 3, with each pair of slits having a slit width that is different from the other pairs of slits. And when a pair of slits 4, 5 is to be used, the stepping motor 1 is rotated through a prescribed angle to align the entrance slit 4 and the exit slit 5 with the light path of the spectrometer.
However, several problems are associated with this type of slit switching mechanism. Namely, even though it is possible to arrange the slit switching mechanism such that the light beam that passes through the slit is able to form images at the position of the slit, because the entrance slits and the exit slits are positioned on the same planar surface that is perpendicular to the output shaft 2 of the stepping motor 1, the incidence light beam and the exiting light beam must form images on that same planar surface. In other words, it is not possible to have the incidence light beam and the exiting light beam form images at different positions. As a result, only a limited arrangement of elements is possible for the spectrometer.
Furthermore, if the light beam that passes through a slit strikes the slit at an angle, it will not be possible to carry out accurate measurements, because the effective slit width will become relatively small in comparison with the actual slit width. Consequently, the light beam must strike each entrance slit and exit slit perpendicular to the slit plate. In other words, the incidence light beam and the exiting light beam must be parallel to each other, and this limits the possible arrangement of elements for the spectrometer.
Moreover, in the slit switching mechanism above, after the stepping motor 1 has been driven to rotate a desired pair of entrance and exit slits 4, 5 into alignment with the light path, the pair of slits must be maintained in that position until all measurements are completed. For this reason, a prescribed excitation current is supplied to the stepping motor 1 in order to generate a holding torque. However, it is difficult to maintain a fixed position for the pair of slits when using a small-sized stepping motor, due to the weak holding torque of small-sized stepping motors. As a result, it becomes necessary to utilize large-scale stepping motors, which in turn necessitates increased dimensions for the spectrometer. Furthermore, since the excitation current must be continually supplied to the stepping motor while the rotation plate 3 is being held at a prescribed rotation angle position, the generation of heat occurs. Besides necessitating the provision of a certain amount of space to dissipate heat, such heat generation makes it impossible to arrange heat-sensitive specimens, light detecting elements and the like in the vicinity of the stepping motor 1. Consequently, the elements of the spectrometer cannot be arranged in a reduced space, and this further increases the difficulty of designing small-sized spectrometers.